Frequency modulation signal detector system



Feb. 8, 1944. :F B, STI-CNE 21,341,484 l [FREQUENCY MODULATIoNsIGNAL DETECTOR SYSTEM f f Filed July s1, 1941 i HMI 1 (1.111 'l m 1111 Y V Gttorneg Patented Feb. 8, 1944 FREQUENCY MoDTJLATioN SIGNAL DETEcToE SYSTEM Fred B. Stone, Haddon Heights, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 31, 1941, Serial No. 404,819

5 Claims. (Cl. Z50-27) i This invention relates to a frequency modulation signal detector system providing means for converting a frequency variation about a mean or center frequency into corresponding amplitude variations, rectifying and amplifying said variations.

Known requency modulation signall detectors commonly employ a frequency discriminator network in connection with rectifying means to convert the frequency modulated signal into an amplitude modulated signal and deriving therefrom the audio frequency modulation.

"It is an object 4of this invention to provide an improved frequency modulation signal detector system wherein the usual discriminator network is eliminated, thereby simplifying the system and wherein a single electric discharge tube of the multi-grid type may be substituted for the discriminator network and rectifier.

The present preferred embodiment of the invention includes an electric discharge tube having a cathode and an anode electrode t.ogether with at least two control grids interposed in the space path of the tube between the anode and cathode electrodes together with a suitable screen or positive electr-ode between the two control grids which are normally at a negative potential.

It is known that a circuit may be provided in connection with a tube having the electrodal arrangement above pointed out such, for example, as that of a commercial type tube known as the RCA 6L? type, which will provide an appreciable D. C. current ow in the first or inner grid circuit, while this grid is at a negative potential, if a signal, such as'an alternating signal, is applied to the third or outer control grid in a high frequency range, for example, of the order of v'l0 megacycles, or higher. This current is caused by the transit time effect of the electron flow through the tube space path.

In accordance with the invention, a suitable impedance, such as a high resistance device, `is placed in circuit with the first or inner grid across which a voltage may develop when a high variable frequency is applied to the third or outer control grid. This has been found to produce a current in the first or inner ,grid circuit through the impedance element which Varies linearly in amplitude with the frequency of the voltage or signal applied to the third or outer control grid. Thus, there is developed in the first control grid circuit a voltage which varies in amplitude at the same rate as the frequency variation of the signal on the third or outer control grid and this voltage variation will appear as a signal in the output circuit, being amplified by the normal gain of theV tube.

It has been found that, as the frequency of the applied signal on the third grid is varied between l5 and 25 mc., the D. C. voltage drop across a high resistance element in the first grid circuit will vary through a range of the order of 1.7 to' 2.5 volts.

If the signal applied to the third or outer grid circuit is varied in frequency at an audio frequency rate, such as a frequency modulated broadcast signal, the demodulated signal or audio frequency component may be derived from the output anode circuit of the detector and suitably amplified in the usual manner for loudspeaker operation.

`While a tuned input circuit for the carrier wave or mean frequency signal may be provided as shown, a tuned circuit is not required for the operation of the system. Thus, a detector of this type is comparatively simple in construction and is readily adjusted in operation.

A tuned circuit as at 6, is not required to obtain resulting conversion of frequency variations to amplitude variations as in other types of frequency modulation detectors. In the present system the conversion of frequency variations to amplitude variations is provided by the relation of the electron transit time effect to the modulation frequency of the applied signal.

It is also a further object of this invention to provide an improved frequency modulation signal detector system which combines therein means for effecting signal amplitude limiting whereby the usual limiter stage preceding the detector may be eliminated, which further simplifies a receiving system embodying the invention.

The invention will, however, be further understood from the following description when considered in connection with the accompanying drawing, and its scope is pointed out in the ac companying claims.

In the drawing, Figure l is a schematic circuit diagram of a frequency modulation signal detector system embodying the invention, and

Figure 2 is a similar schematic circuit diagram showing a modification of the circuit of Figure 1 and also embodying the invention.

Referring to Figure 1, 5 is a frequency modulated signal supply circuit which vis suitably coupled to a signal input circuit 6 through a suitable coupling transformer 1, the secondary 8 of which is tuned by means of a shunt capacitor 9 to a desired mean or center frequency for a signal to be received. The high potential side or terminal I of the circuit 6 is connected to the third or outer control grid I I of a multigrid tube I2 which may be of the mixer -or 6L? type. Thus, in addition to the grid II, the tube is provided with a first or inner grid electrode I3, a screen -grid comprising an element I4 interposed between the grids II and I3, and an element I5 interposed between the grid Il and a suppressor grid I6. The grid elements I4 and I5 are connected as indicated and may be considered as one screen grid electrode.

These grid electrodes are interposed in the space path of the tube substantially in the order shown between the cathodes indicated at I1 and the output anode indicated at I8. As tubes of this character are well known, further description is believed to be unnecessary.

The screen grid III- I5 is operated at a positive potential, as indicated, while the control grids II and I3 are operated at negative potentials, in the present example being connected to the ground or negative end of a self-bias resistor in the cathode circuit. The grid II is connected with this bias source through a ground connection 2l for the low potential side of the circuit 6, and the bias connection for the grid I3 is completed through an impedance element comprising a high resistance resistor indicated at 22. The resistor 20 is provided with an audio frequency bypass capacitor 23 in shunt therewith, while the resistor 22 is provided with a high frequency bypass capacitor 24 in shunt therewith. The suppressor grid I5 is connected to the cathode as indicated.

The audio frequency or modulation output circuit 25 is connected with the output anode I8, as indicated, and provides the modulation component of the signal which is normally at audio frequency, asindicated.

The audio frequency output circuit is further provided with an output impedance comprising a plate resistor 21, which may be energized at a positive potential, as indicated, and this is provided also with a bypass capacitor 28, which is effective only at the signal frequency, as is also the capacitor 24.

The resistor 22 is the detector output circuit impedance element, since a variable frequency voltage applied to the grid IIV causes a current to flow in the circuit of the grid I3, the amplitude of which current varies with the frequency of the voltage applied to the grid II. This current flow in the grid current of the grid I3 develops a modulation frequency voltage across the resistor 22. The conversion of frequency variation can therefore be seen to occur in the circuits comprising grid electrodes II and I3, `and the resistor 22.

The modulation of audio frequency voltage across the resistor 22 is applied between the grid electrode I3` and the cathode, and appears across the audio frequency output impedance 21 being amplified by the effective gain of the tube from the grid electrode I3 to the plate electrode I8.

The signal grid circuit of the detector may be provided with a suitable grid leak resistor and grid capacitor 3| which, in the present example, are shunt connected and serially included in circuit with the grid I I. A switch 32 provides a short circuit across the elements 30 and 3| for rendering them ineffective in the grid circuit when so desired.

The operation of the system described is as follows:

When a frequency modulated carrier wave or signal is applied to the input circuit 6 from the source 5, assuming the modulation to be sound or audio frequencies, a D. C. current which varies in amplitude with the frequency variation of the signal applied to the signal grid II, is set up in the circuit of the inner or rst grid I3 across the impedance 22. This voltage B across the impedance or resistor 22 then varies inv amplitude at the same rate as the frequency variation of the signal applied to the signal grid II, and this voltage or signal, which is the audio frequency signal, is amplified by the normal gain of the tube I2 and appears in the output circuit 24 across the output impedance 21. Thus, the frequency modulation signal is converted to an amplitudev modulated signal which, in turn, is detected and amplified all in one stage or detector circuit involving a single multigrid tube. No discriminator network is required and no balanced detector or rectifier circuit is provided. Thus the system is not only of low cost construction, but is readily adjustable in operation for effective frequency modulation detection.

By inserting in the signal grid circuit a suitable network, such as the resistor 30 and the capacitor 3| providing a grid leak and capacitor, a grid leak limiting action is combined with frequency modulation detection, and this effect in the present circuit is obtained by opening the switch 32.

A circuit arranged for adjusting the polarizing potentials of the tube electrodes is shown in Fig. 2 to which attention is now directed. In this circuit the same reference numerals are applied to like circuits and elements as in Fig. 1. 'I'he grid return connection 35 for the signal or control grid II is provided by a suitable tap 36 on the self-bias resistor 20 as a bias voltage source for the grid II and, likewise, the suppressor grid connection 31 is returned to a tap 38 also on the resistor 2l), both connections being suitably bypassed, as indicated at 39, for audio as well as signal frequencies.

It should be noted that the inner or rst control grid is also connected with a source of negative biasing potential provided by the negative section 40 of a bleeder resistor 4I having positive and negative supply terminals 42 and 43. The impedance 22 is connected with a tap 44 on the negative portion of the bleeder resistor to a supply lead 45. The ground terminal of the resistor is indicated at 46. The screen grid III-I5 is connected with a tap 41 suitably bypassed by a capacitor 48, and the anode supply circuit through the rsistor 21 is connected With a positive terminal With this arrangement, the various tap connections may be made at suitable voltage points for applying to the grid electrodes the proper polarizing voltages to establish a saturation effect in a tube of proper design, for a given amplitude of the frequency modulated carrier or signal applied to the signal or outer grid I I.

The polarizing voltages applied to control grid electrodes I I and I3 and to screen grid electrodes I4 and I5 are adjusted so as to produce the optimum conversion of frequency variations to amplitude variations in the frequency modulation detector section of the electron discharge device and at the same time adjusted so as to obtain good audio frequency amplifier performance from the sale electron discharge device considering grid electrode I3 and plate electrode I8 as the audio frequency amplifier input and output electrodes.

The signal input grid II is operated at asignal lever such that it draws grid current and therefore any change in the amplitude level at the input circuit 6 does not result in appreciable change in the signal level applied between the grid electrode Il and the cathode. Amplitude variations present in the signal developed across the input circuit 6 therefore produce substantially no variation in the amplitude of the current owing in the circuit of the grid electrode I3 and hence in the output voltage.

A frequency modulation signal detector system in accordance with the invention is greatly simpliiled in that there are no tuned circuits required in the operation, and the alignment problem of other known types of frequency modulation detectors is avoided. Furthermore, the range of linear operation is practically infinite as compared with the linear range obtained by the use of tuned circuits of known frequency modulation detectors. This is an advantage in systems employing a large frequency swing.

I claim as my invention:

1. A frequency modulation signal detector system comprising in combination an electric discharge tube having an anode, a cathode, and two interposed control grids, means for applying operating potentials to said grids to establish a predetermined transit time for the electron ow through said tube whereby one of said control grids draws a grid current which varies linearly in amplitude with the frequency variation of an applied frequency modulated signal on the other of said control grids, a resistor and a shunt bypass capacitor providing the sole impedance element in circuit with said first-named grid for developing across said resistor a modulation frequency voltage resulting from said current flow on said grid, means for applying a frequency modulated signal to the other of said grids, and means for deriving a modulation signal component of the applied signal from said anode.

2. A frequency modulation signal detector system comprising in combination an electric discharge tube having an anode, a cathode, a screen grid, and two control grids interposed in the space path of the tube with the screen grid therebetween, means including a signal input circuit for applying frequency modulated signals to the control grid more adjacent to the anode, a modulation frequency impedance element in circuit with the other control grid, means for applying operating potentials to said grid electrodes for developing a grid current through said impedance which varies linearly in amplitude with the frequency variation of an applied signal on the first-named control grid, and a signal output circuit connected with the anode for deriving therefrom amplified signals from the other of said grids in response to current variations through said impedance.

3. A frequency modulation detector system comprising in combination an electric discharge tube having an anode, a cathode, and two interposed control grids, means for applying a frequency modulated high frequency signal to the outer one of said grids, means for operating said grids at potentials with respect to the cathode whereby the other of said grids tends to develop a grid current which current varies linearly in amplitude with the frequency variation of the applied signal, means providing an audio frequency' impedance in circuit with the other of said grids for developing the modulation signal voltage on said grid, and a signal output circuit connected with the anode for deriving therefrom amplified signals from said last-named grid circuit increased in amplitude by the effective amplification from said other grid to said anode.

4. 9A frequency modulation signal detector systern comprising in combination an electric discharge tube having an anode, a cathode, a screen grid and two control grids having at least a portion of the screen grid interposed therebetween, the rst of said control grids being more adjacent to the cathode and the second of said control grids being more adjacent to the anode, means for operating said grids at potentials with respect to the cathode such that the first of said control grids tends to develop a grid current which varies in amplitude linearly with the frequency variation of an applied signal on the second control grid, means for applying a frequency modulated signal to the second control grid, means providing a modulation frequency impedance in circuit with the irst control grid for developing a modulation frequency signal voltage on said grid across said impedance, a signal output circuit connected with the anode for deriving therefrom the modulation component of a received frequency modulated signal, and means in circuit with said tube for effecting signal amplitude limiting therethrough.

5. A frequency modulation signal detector system comprising in combination an amplifier tube having a cathode, an anode, a rst control grid, a screen grid, and a second control grid disposed in the space path of the tube between the cathode and anode in the order named, means for applying a frequency modulated signal between the cathode and the second control grid, means for applying operating potentials to said tube for controlling the transit time of the electron flow therethrough whereby the first control grid develops a grid current which varies linearly with the frequency variation of an applied frequency modulated signal on the second control grid, a resistor in circuit with said first control grid as an impedance element for developing a modulation voltage thereon in response to said grid current, and means providing a modulation signal output circuit between the cathode and the anode.

FRED B. STONE. 

